The present invention relates to amplifiers, and more particularly to amplifiers including apparatus for eliminating distortion caused by noise on the DC power supply line, including slow variations in the DC supply voltage, AC ripple and audio signals coupled through the power supply line.
It is known that noise appearing on a DC power signal applied to the power input of an amplifier can and will be coupled to the output of the amplifier as hum, intermodulation distortion, and noise. Power supply noise is of particular concern in high power amplifiers, such as those found in the transmitters conventionally used in commercial AM broadcasting. DC power supplies are, therefore, designed to reduce as far as possible the magnitude of the noise present on the DC power supply lines.
Typically, small de-coupling capacitors having low impedance at high frequencies are connected across the power supply output to reduce high frequency noise on the power supply line. Large capacitors (usually electrolitic) are connected across the power supply to reduce low frequency DC supply voltage variations and AC ripple. In theory, the level of low frequency noise could be reduced to any desired degree by connecting a suitably large amount of capacitance across the power supply output. Unfortunately, the low frequency filter capacitors used in high power DC power sources are quite large both in volume and in weight. The inclusion of further power supply filtering capacitors is undesirable both due to the cost of such capacitors, and to their size.
Another known method of combating power supply noise effects is through the use of negative feedback. Noise in the output signal is detected, and is feedback to the input of the amplifier in such a fashion as to reduce the overall magnitude of the noise. The feedback signal is combined with the input signal in an additive combiner circuit. Feedback systems are undesirable both due to the difficulty of isolating the noise, and to the instability problems inherent in any feedback system.